Packing containers fabricated from flexible packing material, e.g., paper and plastics laminate, exist in a great number of forms and are used, among other things, for the packaging of liquid foodstuffs, e.g., milk and juice. A packing container of this type is the so-called gable-top package which comprises four side walls, a plane bottom and a rooflike top part which has main panels inclined towards one another, whose upper parts are sealed together to a sealing fin.
In the type of packing container described above, it is possible in general to open portions of the top part and form them to a pouring spout. The opening arrangement itself may be designed in a number of different ways, but usually the actual top seal between the upper ends of the different wall panels or the folds included in the top is utilized as an opening arrangement. When the packing container is to be opened, the top seal produced in the manufacture of the packing container is then broken so that some of the panels folded inwardly can be folded out in order to form a pouring spout. The openability of the packing container consequently will depend to a great extent on the strength of the top seal which means in practice that a choice has to be made between a good, and thus tight, top seal and an easily openable but less tight top seal. To eliminate this problem it has been proposed to separate the sealing and opening function by providing the packing material at a suitable place with another type of opening arrangement, e.g., a perforation line. This, however, means a more complicated manufacturing process and, since the perforation line has to be placed below the top seal of the packing container, the opening arrangement gets close to the liquid surface in the packing container which is a disadvantage upon opening of the packing container as well as during pouring out of the liquid contents from the packing container.
The original design with a combined top seal and opening arrangement is generally preferred, and it has been attempted, therefore, in recent times to optimize this design to obtain a good seal and tightness as well as good openability. Since the top of the packing container usually is heat-sealed by fusing together layers of thermoplastic material situated on the outside of the packing material, several of these attempts were based on a reduction of the sealing strength within the limited region of the top seal, which is used in the opening of the packing container while at the same time maintaining the sealing temperature and the sealing pressure. This reduction of the sealing strength may be achieved, for example, by providing the packing material within the region which is to be readily openable with an uneven surface, so that the total adhesion area within this region is reduced (EP-A-185.325). However, this technique has proved to be subject to certain disadvantages and it has been found difficult in practice to obtain the predetermined, desired sealing strength. This disadvantage can be overcome with the help of another known technique (EP-A-270.869), according to which the region of the top seal which is to be opened is provided with a coating of a seal-preventing means which can be applied to the packing material in advance by means of a printing process. This technique provides a readily openable packing container which, however as a result of certain combinations of seal-preventing means and plastic coatings on the packing material may cause the packing containers to leak. This problem is accentuated especially in packing containers which are used for aseptic contents. For this type of contents and packing container the demands on tightness are extremely high, since even a so-called microleakage impairs the sterile conditions and reduces the keeping properties of the product to an appreciable degree.
Another known technique (U.S. Pat No. 4,712,727) uses laminated pieces of strip which include layers of an adhesive and which are placed on the region of the blank where a limited closing force is desired, that is to say at the top on both sides of the folded-in pouring spout. However, the handling and placing of the individual pieces of strip in the correct position on the blank is difficult to master and the method, moreover is relatively expensive.
It is evident from the above that it has not been possible up to now to provide a packing container of the gable-top type which has a top seal that is easily openable when an opening of the packing container is intended and which is completely tight. This demand has been particularly difficult to meet in aseptic packing containers, since even very small channels through the top seal may cause the sterility to be adversely affected, so that the keeping properties of the product no longer can be guaranteed.